1. Field of the Invention
The present invention relates to a lightwave interference measurement apparatus (interferometer) that measures an absolute distance using lightwave interference.
2. Description of the Related Art
As a conventional lightwave interference measurement apparatus that measures an absolute distance, a wavelength-scanning lightwave interference measurement apparatus is known. Commonly, since the absolute distance measurement by the wavelength scanning is a low accuracy measurement, a method of combining a relative distance measurement by a fixed wavelength with it to improve the accuracy is used. In these method, an accuracy of an amount of wavelength scanning, an accuracy of the fixed wavelength, a phase measurement accuracy at the time of measuring the relative distance are main accuracy factors.
Japanese Patent No. 2725434 discloses an FM heterodyne method that measures a single interference signal intensity to calculate an absolute distance based on the intensity change of the interference signal that is generated by the wavelength scanning. Japanese Patent No. 2810956 discloses a method of introducing a phase measurement by a Lissajous waveform based on two interference signal intensities having phases different by 90 degrees from each other as a method of a highly-accurate absolute distance measurement compared with the FM heterodyne method.
However, in the conventional lightwave interference measurement apparatus, in order to obtain sufficient measurement accuracy by combining the absolute distance measurement and the relative distance measurement, a large amount of wavelength scanning was necessary. There are problems to realize the large amount of wavelength scanning and measure the absolute distance at high speed as follows. When an inexpensive DFB laser is used, the large amount of wavelength scanning can be performed by the wavelength scanning by the temperature modulation, but the wavelength scanning cannot be performed at high speed because of taking much time to follow it. In order to solve the problem, there is an external resonator-type LD or the like that is capable of performing the wavelength scanning at high speed by adjusting the tilt of the grating, but this is expensive. Furthermore, since an interference order cannot be determined when a surface under test (test surface) varies, a complicated process that stabilizes the surface under test was necessary in order to perform the absolute length measurement.